This invention relates generally to a system and method for planning and scheduling work flow and processes for reconfigurable production operations and equipment, which, depending upon the job, may be variably applied to work pieces that may be highly variable between different jobs.
Reconfigurable production systems increasingly consist of multiple parallel, alternative modules that are connected through flexible paths and even loops. Consequently, such systems are expected to offer a multitude of alternative operations (or capabilities) to produce the same outputs. For example, a modular printing system may consist of several identical, parallel printers connected through flexible paper paths that feed to and collect from these printers. The system control software available for such systems today has as its heart a sophisticated scheduling component, but is not set up to make good use of such a reconfigurable, parallel production system. This is primarily due to the lack of a generic, powerful planning component which would select the optimal sequence of operations among the plurality of available operations.
A reconfigurable production system may be modeled as a graph of connected modules, with each module described by a model of its structure and its capabilities. The structure is primarily the interface through with work units enter and exit, such as entry and exit ports, plus any internally used resources. A capability is an operation that accepts work units at entry ports, processes them, and moves them to exit ports. (Entry and exit ports here refer to mechanical interfaces, such as slots or trays, as well as computer interfaces. A port may serve as both entry and exit port.) Operation of such a system has been modeled as a sequence of capability executions as work units move along valid paths in the graph from module to module.
An example for a reconfigurable production system is a modular printer, with modules such as feeders, mark engines, paper transports, inverters, etc. There, the work units are sheets and images. A simple paper transport module has an entry port, an exit port, and a single capability, to move a sheet of paper from its entry port to its exit port. An inverter module has one entry port, one exit port, and two capabilities, one to invert a sheet of paper and one to bypass the inversion mechanism. A mark engine transfer module has two entry ports (one for sheets and one for images), one exit port (for marked sheets), and one capability, to print the image onto the sheet. A sample resource in all of these modules is the space occupied by the sheet, which may only be occupied by one sheet at a time. Other examples of reconfigurable production systems are assembly lines, for example for the assembly or packing of computer parts, and automated analytic systems, such as blood sample analysis machines. In these different production systems, work units may be sheets of paper, electronic files, computer parts, semiconductor wafers, blood sample trays, any parts or composites of these, or other physical or electronic objects being processed by production systems. Transport mechanisms may be conveyor belts or robotic arms or any other devices or functions for moving work units.
Module capabilities may be composed to system capabilities by incrementally unifying work unit and time variables of output and input events at connected modules along valid paths in the system graph. For example, if a module's exit port is connected to another module's entry port, any capability producing work units for the first module's exit port potentially can be composed with any capability consuming work units from the second module's entry port. Unification of work unit and time variables ensures the consistency of attribute and time constraints.
A scheduler for such systems receives a stream of jobs, each consisting of a sequence of desired work units to be produced at some final exit port of the system. Each desired work unit is described by a work unit variable with attribute constraints. This is used to select a suitable system capability that can produce the desired work unit by unifying the desired work unit variable with the work unit variables of system capabilities producing work units for the desired exit port. As system capabilities for the desired work units in the jobs are found, their time and resource constraints are posted to the constraint store, and the constraints are solved in order to find time values for the various module capabilities producing the desired work units. The selected module capabilities plus the time values are then sent to the modules so that they can execute the corresponding operations at the designated times.
This approach to system control does not have a real planning component. Instead, the controller selects the first system capability it can find and schedules that capability. Also, the selection requires that the output, such as the finisher where the job should be delivered, be specified. These restrictions are acceptable in some production systems, such as a typical in-line printing system that has either few alternative system capabilities for the same kind of job, or where alternative system capabilities mainly differ in the feeders and finishers being used.
This approach proves unsatisfactory for systems with many alternative, parallel system capabilities. For example, printing systems with multiple identical or similar mark engine modules require a more intelligent selection of the capabilities to achieve optimal productivity and load balancing. This selection can in principle be done before the scheduling step based on rules (for example, a rule for alternating between all mark engines to attempt optimal load balancing). It is preferable, however, to integrate planning with scheduling, in which the selection of individual capabilities would depend on the selection and timing of other capabilities being planned and scheduled at the same time.